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  Composite tube, method of producing for a composite tube, and use of a composite tubeUSPTO Application #: 20080014342Title: Composite tube, method of producing for a composite tube, and use of a composite tube Abstract: To provide a tube which is particularly well matched to the specific demands imposed in special application areas, such as for example hydropyrolysis, the invention proposes a composite tube having a first part-tube and a second part-tube, in which—one part-tube is arranged in the other part-tube,—the first part-tube is a centrifugally cast tube, and—the second part-tube has been produced by pressure treatment from a powder. (end of abstract)
Agent: Henry M Feiereisen, LLC - New York, NY, US Inventors: Dietlinde Jakobi, Hans-Peter Duster, Carlos Marturet USPTO Applicaton #: 20080014342 - Class: 427181000 (USPTO) Related Patent Categories: Coating Processes, Solid Particles Or Fibers Applied, Interior Or Hollow Article Coating The Patent Description & Claims data below is from USPTO Patent Application 20080014342. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCES TO RELATED APPLICATIONS [0001] This application is a continuation of prior filed copending PCT International application No. PCT/EP2005/008813, filed Aug. 12, 2005, which designated the United States and has been published but not in English as International Publication No. WO 2006/018251 and on which priority is claimed under 35 U.S.C. .sctn.120, and which claims the priority of German Patent Application, Serial No. 10 2004 039 356.7, filed Aug. 12, 2004, pursuant to 35 U.S.C. 119(a)-(d), the contents of which are incorporated herein by reference in their entirety as if fully set forth herein. BACKGROUND OF THE INVENTION [0002] The invention relates to a composite tube, to a process for producing a composite tube and to uses of a composite tube. [0003] Tube furnaces in which a hydrocarbon/steam mixture is passed through a series of individual or meandering tubes (cracking tube coils) at temperatures of above 750.degree. C. made from heat-resistant chromium-nickel-steel alloy with a high resistance to oxidation or scaling in flue gases and a high resistance to carburization have proven suitable for the high-temperature pyrolysis of hydrocarbons (crude oil derivatives). The tube coils comprise, for example, vertically running, straight tube sections which are connected to one another via U-shaped tube bends; they are usually heated with the aid of side-wall burners and in some cases also with the aid of bottom burners and therefore have what is known as a light side, facing the burners, and what is known as a dark side, which is offset by 90.degree. with respect thereto, i.e. runs in the direction of the rows of tubes. The mean tube metal temperatures (TMT) are in some cases over 1000.degree. C. [0004] The service life of the cracking tubes is dependent to a very significant extent on their carburization resistance and this in turn is dependent on the coking rate. A crucial factor for the coking rate, i.e. the growth of a layer of carbon deposits (pyrolysis coke) on the tube inner wall is, in addition to the type of hydrocarbons used, the cracking gas temperature in the region of the inner wall and what is known as the operating severity, which conceals the influence of the system pressure and the residence time in the tube system on the ethylene yield. The operating severity is set on the basis of the mean outlet temperature of the cracking gases (e.g. 850.degree. C). The higher the gas temperature in the vicinity of the tube inner wall above this temperature, the more extensive the growth of the layer of pyrolysis coke becomes, and the insulating action of this layer allows the tube metal temperature to increase still further. Although the chromium-nickel-steel alloys containing 0.4% of carbon, over 25% of chromium and over 20% of nickel, for example 35% of chromium, 45% of nickel and if appropriate 1% of niobium, that are used as tube material have a high resistance to carburization, and carbon diffuses into the tube wall at defects in the oxide layer, where it leads to considerable carburization, which can amount to carbon contents of from 1% to 3% at wall depths of 0.5 to 3 mm. This is associated with considerable embrittlement of the tube material, with the risk of crack formation in the event of fluctuating thermal loads, in particular when the furnace is being started up and shut down. [0005] To break down the carbon deposits (coking) on the tube inner wall, it is necessary for cracking operation to be interrupted from time to time and for the pyrolysis coke to be burnt with the aid of a steam/air mixture. This requires operation to be interrupted for up to 36 hours, and therefore has a considerable adverse effect on the economics of the process. [0006] British patent 969 796 has disclosed the use of cracking tubes with inner fins. Although inner fins of this type result in an internal surface area which is a good percent, for example 10%, larger, with a corresponding improvement in the heat transfer, they are also associated with the drawback of a considerably increased pressure loss compared to a smooth tube, on account of friction at the enlarged tube inner surface. The higher pressure loss requires a higher system pressure and therefore has an adverse effect on the yield. An additional factor is that known tube materials with high carbon and chromium contents can no longer be profiled by cold-working, for example cold-pressing. They have the drawback that their deformability decreases greatly as the hot strength and the resistance to carburization and oxidation increase. This has led to the high tube metal temperatures of, for example, up to 1050.degree. C., which are desirable with regard to the ethylene yield, requiring the use of centrifugally cast tubes. [0007] In centrifugal casting, the molten alloy is cast into the end of a tubular casting mould which rotates at such a high velocity that the molten alloy forms a layer of liquid alloy on the inner side of the casting mould. After the alloy has solidified, the rotation of the casting mould is stopped and the tube which has been formed in this way can be ejected. The tube is drilled out over its length in order to have the required internal diameter. Any oxide impurities will always be lighter than the alloy and will therefore "float" on the inside of the tube, with the result that they are removed by the drilling. [0008] However, since centrifugally cast tubes can only be produced with cylindrical walls, a special cutting or electrolytically material-removing machining operation is required to produce an internally finned tube. [0009] European patent EP 0 980 729 B1 describes this type of electrolytic machining of a centrifugally cast tube. For this purpose, the tube blank is introduced in to a holding device which is sealed all the way around at its open ends. The sealing only allows an electrolyte to flow in and out and an electrode bar, which has an electrode attached to its end, to pass through; the electrode can be moved by means of the electrode bar along the inside of the tube to be machined, in the axial direction of the tube. On its outer surface, the electrode has a series of peaks and valleys. The material of the inner side of the tube is electrolytically removed by a voltage difference being applied between the electrode and the tube via electrical terminals, which are arranged spaced apart along the tube, and via a current connection block at the end of the electrode bar. In this way, the tube interior is provided with a profile of the geometric shape of the outer surface of the electrode. However, this process has proven complex to carry out. [0010] The production of composite tubes by centrifugal casting, with the part-tubes produced in separate machining steps and being metallurgically joined to one another, is known from U.S. Pat. No. 6,406,800 B1, which describes a tube bend for pipelines for transporting solids. The composite tube used as the starting product for the bent tube is produced by centrifugal casting. First of all, the material which forms the outer tube is poured in the molten state into the casting mould, which rotates at a high speed, so that the molten alloy forms a layer of liquid alloy on the inside of the casting mould. Shortly before the alloy completely solidifies or immediately after complete solidification, the molten alloy which forms the inner tube is likewise poured into the rotating casting mould, so that the molten second alloy forms a layer of liquid alloy on the inside of the virtually solidified first alloy. The two materials are mixed in the transition region between outer tube and inner tube and thereby produce a metallurgical join between the two tubes. The alloys described in U.S. Pat. No. 6,406,800 B1 are not suitable for use in high-temperature pyrolysis. A further drawback is that only alloys which are suitable for centrifugal casting can be used for the inner and outer tubes. [0011] Furthermore, it is known from U.S. Pat. No. 5,069,866 to produce a tube having an outer tube and an inner tube joined to the outer tube by hot isostatic pressing (HIP) from two powders. The austenitic steels or nickel alloys described in that document, however, are likewise not suitable for use in high-temperature pyrolysis. SUMMARY OF THE INVENTION [0012] In view of this background, the invention is based on the problem of proposing a tube which is particularly well matched to the specific demands imposed in special application areas, such as for example hydropyrolysis. Furthermore, it is intended to propose a process for producing tubes having an inner tube and an outer tube. [0013] According to one aspect of the present invention, this problem is solved by a composite tube having a first part-tube and a second part-tube, wherein one part-tube is arranged in the other part-tube, the first part-tube is a centrifugally cast tube, and the second part-tube has been produced by pressure treatment from a powder. According to another aspect of the present invention, this problem is also solved by a process for producing a composite tube, wherein a powder is brought into contact with the inner or outer surface of a centrifugally cast tube, and the powder is compacted by pressure treatment to form the second part-tube and joined to the centrifugally cast tube. [0014] The invention is based on the underlying concept of forming a composite tube having a first part-tube and a second part-tube, in which one part-tube is arranged in the other part-tube, the first part-tube is a centrifugally cast tube, and the second part-tube has been produced by pressure treatment from a powder. By combining two part-tubes, it is possible for the tube according to the invention to be specifically matched to the demands imposed in special application areas, such as for example hydropyrolysis. [0015] The second part-tube can be used in particular to improve the corrosion properties, even at temperatures of up to 1200.degree. C., the wear resistance, the carburization and coking behaviour when used in ethylene crackers and the heat transfer of centrifugally cast tubes. [0016] For use in the conveying of highly corrosive media, it is possible to use a centrifugally cast tube made from a corrosion-resistant material having, for example, limited mechanical properties, while the remainder of the wall thickness is made from a less expensive material with good mechanical properties. [0017] On account of its homogeneous microstructure and the possibility of using chromium-nickel steels, a centrifugally cast tube is particularly suitable for the thermal cracking of hydrocarbons. However, by forming a second part-tube, which is produced from a powder, for example by hot isostatic pressing, it is possible to impart further properties to the centrifugally cast tube which are unattainable by such a tube itself. For example, the second part-tube can be made from materials which are not suitable for a centrifugal casting process but can be produced in powder form. Furthermore, the second part-tube can be provided with a geometry, for example a surface profile, during the pressure treatment of the powder. With pure centrifugally cast tubes, geometries of this type can otherwise only be achieved by complex remachining steps, for example a cutting or electrolytically material-removing remachining operation. The composite tube according to the invention may, however, also have a smooth surface. In this case, the advantages of the wider choice of materials which are made possible by the use of a powder come to the fore. Under certain circumstances, it is also possible to dispense with the expensive further treatment of the centrifugally cast tube, for example the drilling operation, for the centrifugally cast tube to be used in the composite. [0018] A composite tube is to understood as meaning a tube which, based on its cross section, has at least two regions (part-tubes), which differ from one another by virtue of the way in which they are produced. [0019] A pressure treatment of the powder is to be understood as meaning any compacting of the powder which, if appropriate in combination with heating of the powder, produces a cohesive solid from the powder or a pre-compacted powder. It is particularly preferable for the second part-tube to be produced by means of hot isostatic pressing (HIP). [0020] In a preferred embodiment, the first part-tube is arranged so as to directly adjoin the second part-tube, as seen in the radial direction of the composite tube, and one of its surfaces is fixedly joined to a surface of the second part-tube. However, it is also possible to provide further part-tubes, in particular further centrifugally cast tubes or further part-tubes produced by pressure treatment from a powder. It is in this way possible to produce various layers of a composite tube which have preferred properties for their particular position in the tube cross section. [0021] In a preferred embodiment, the second part-tube has a profile, in particular one or more internal fins, the profile may also be an external profile. A profile can be used to influence media flowing inside the composite tube according to the invention or media flowing along the outside of the composite tube, for example by swirling them up. The particularly preferred embodiment with internal fins is eminently suitable for use in a process for the thermal cracking of hydrocarbons in the presence of steam. Continue reading... Full patent description for Composite tube, method of producing for a composite tube, and use of a composite tube Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Composite tube, method of producing for a composite tube, and use of a composite tube patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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